1. Field of the Invention
The present invention relates to an Analog-To-Digital converter having a pipeline architecture.
2. Description of the Related Art
Analog-To-Digital (ADC) converters are commonly used in a wide variety of applications (for example, in the telecommunication field), whenever an analog signal is to be converted into a corresponding digital signal. For this purpose, many kinds of converters have been proposed in the last years. In a particular architecture, known as pipeline or multistage, the converter uses a series of stages providing successive approximations of the digital signal.
Particularly, each stage performs a low-resolution conversion and produces a sub-set of the desired bits of the digital signal. A residue of the analog signal (representing a quantization error of the conversion) is then passed to a next stage in the pipeline; the next stage generates a further sub-set of lower significant bits of the digital signal, and so on until the last stage of the pipeline. This architecture provides high resolutions, using very simple and inexpensive stages.
Typically, the residues are amplified by a pre-set analog gain before being passed to the next stages; in this way, each stage operates with a similar input signal range. However, any error in the (inter-stage) gain causes a harmonic distortion in the digital signal generated by the converter.
This problem is particular acute in the first stages of the pipeline (since the corresponding error in the inter-stage gain is amplified by all the next stages). The inherent imprecision of the inter-stage gain (due to the limits of the technological process used to implement the converter) then strongly reduces the actual resolution that can be achieved.
For example, a converter at 14 bits with stages at 1 bit (wherein the inter-stage gain is 2), would require a precision in the inter-stage gain of the first stage equal to ½13 ρ0.012%; this precision is substantially impossible to achieve, particularly when the converter is integrated in a chip of semiconductor material (or in any case it would require very expensive manufacturing techniques, such as laser trimming processes).